[volt-nuts] Traveling Standards

[Andreas Jahn]

Hello Bob,

whats the matter with you. Infected by precision virus like me?
You wanted to have a standard with about 10ppm and now you blame a 3-4ppm 
drift.

>
> The LM199A is hermetically sealed.
>

The PCB, the 8K Resistor and the voltage Regulator are not.

> Although a small sample, the two references appear to be similarly 
> affected by
> whatever caused the drift - similar range of drift, similar time constant.
>
On the first view I would blame it on the meter.
It is very unusual that the drift of 2 different references has nearly exact 
the same amount of ppm and direction.
But on the other side you state that there are several HP3458A which 
recorded the drift.
It is not probable that all came freshly from calibration of a other 
location.

> Any ideas about what could cause the drift we are seeing?

>From time constant it could be the humidity change.

My 2 LT1027CCN8-5 references which ara mounted only with 1 Pin
to the PCB have time constants in the range of 4-7 days.
The epoxy material of a PCB should lie in the same ball park area.
The LT1027 are influenced by around 0.5 ppm per percent humidity change.

For the hermetically sealed brand new references LT1236AILS they state in 
their new product catalog
a humidity change of less than 10ppm for 25% humidity change. (page 36)
http://cds.linear.com/docs/Product%20Info/NPC.pdf
I asked them whether this is from mechanical stress from the PCB and they 
confirmed to me
 that with a dead bug mounting the influence of humidity will be virtually 
unmeasurable.
So they will delete the parameter from the data sheet.

So for the LM399 it might be mechanical stress introduced by the PCB.


When looking at your cirquit there are several points to mention:
One common failure source will be the LM78L15. A output voltage change will
influence the supply of MAX6350 and the reference current of LM399.
PSRR of MAX6350 is about 2-5 ppm/V above 10V supply. (without self heating 
effects).

The LM399 resistor will give a current change of about 10% per Volt (100uA)
resulting with 0.5 Ohm impedance in about 50uV/V or 7ppm/V

Other weak points of the cirquit are:
The LM399 heater voltage is not stabilized. this will give about 0.5ppm/V

And finally: was the LM399 always in the same orientation during 
measurements?
(will be difficult with a cylindrical housing).
My LM399 drift 3-4 ppm by tilting orientation.

With best regards

Andreas